1. Field of the Invention
The present invention relates generally to an improved system for handling coiled tubing on an offshore platform or installation. More specifically, the invention is a system for reducing the load (i.e., stack weight, tubing load, etc.) on the wellhead and for providing a flexible connection between the wellhead and the coiled tubing stack.
2. Description of the Prior Art
There are three broad classes of offshore installations: floating platforms or floaters, fixed leg, and tension leg. Floating platforms are connected only to the sea floor by a marine riser. Fixed leg platforms have solid legs that reach all the way to the sea floor. Tension leg platforms (including spars) have cables that pull the buoyant structure deeper into the water.
Operations on fixed leg platforms are very similar to land operations. The only significant differences are that the work space is more limited and all of the equipment must be delivered or transported to the platform. Typically, such delivery is by boat, with the equipment being lifted into place with the platform crane.
Operating on floaters requires that the coiled tubing equipment be placed in the load path of the marine riser. This requires that a lifting frame capable of carrying 350 tons (for a typical operation) is required to allow the injector and BOPs to be isolated from this enormous load. The key challenges are getting this frame into the derrick, rigging up the coiled tubing equipment in it and connecting to the wellhead/marine riser. Once everything is rigged up, the marine riser is attached to the wellhead on the sea floor. From this point on, the floater is moving up and down with respect to the injector, with the motion being compensated for by the derrick blocks. The floater is dynamically positioned, so the riser stays vertical.
Operations on tension leg platforms (TLPs) require set-up/rig-up operations which fall between those required for floaters and fixed leg platforms. There are decks to rig up on, but the wellheads are connected to the sea floor with risers. A tensioning system ensures that this riser is always being pulled upward (to prevent it from buckling and/or being lost underwater). As the TLP is moved by waves, wind and currents, the wellhead moves relative to the TLP. On some TLPs this motion is constrained to be perpendicular to the deck. On others the wellhead pivots about a spherical joint and moves vertically. The details of the motion depend on the construction of the platform. In general, the wellhead on an off-shore platform is unable to support significant additional load (unlike typical wellheads on land). Wellhead motion, during storms for instance, can range as high as six feet and six degrees of tilt. Also, for platforms using buoyant cans to support the wellhead, the loss of one or more cans can cause the wellhead to sink as much as twenty feet. The primary challenges for this sort of platform include handling the wellhead vertical motion, angular motion, and lack of load carrying ability.
Lifting frames that carry the marine riser load around the coiled tubing equipment are currently available. Common problems with these frames include difficult rig up and difficulty getting them through the V-door and into the derrick.
For operations where the wellhead can carry the tubing load and does not tilt, compensating jacking frames may be employed. These frames prevent the coiled tubing stack from falling over and allow it to move up and down in response to the vertical motion of the wellhead.
Yet another option for handling wellhead movement is a hook load compensator. This system allows an offshore job (on a wellhead that can support load and that does not tilt) to proceed like a land job. The coiled tubing stack is attached at the top to a device that transfers force between the top of the stack and the hook of a platform crane, but still allows motion. Currently available devices do this using one or more hydraulic cylinders and hydraulic accumulators. As the cylinder is pulled out, it compresses the gas in the accumulator, increasing the force carried by the cylinder.